Dehydration catalysts, particularly for the dehydration of diols

ABSTRACT

Diol dehydration catalysts and methods of preparing and using the same, constituted at least in part of a pyrophosphate of at least one metal belonging to the group consisting of lithium, sodium, strontium, and barium and possibly of at least one orthophosphate of said metals and including chromium in combined form and/or a basic additive.

United States Patent 1 1 Weisang et a1.

1 1 DEHYDRATION CATALYSTS,

PARTICULARLY FOR THE DEHYDRATION OF DIOLS [75] Inventors: Joseph-EdouardWeisang; Georges Szabo, both of Le Havre; Jean Maurin, Montivilliers,all of France [73] Assignee: Companie Francaise De Rat'finage,

Paris, France [22] Filed: July 16, 1973 [21] Appl. No.1 379,820

Related US. Application Data [63] Continuation-impart of Ser. No.132,892, April 9,

1971, Pat. No. 3,781,222.

[30] Foreign Application Priority Data 1451 Jan28, 1975 2,300,12310/1942 Keunecke et al. 252/437 X 2,362,311 11/1944 Rugin 252/437 X2,367,877 1/1945 Layng 252/437 2,370,472 2/1945 King 423/312 2,426,6789/1947 Greenberg..... 260/681 X 2.511249 6/1950 Durgin et a1... 423/3062,901,400 8/1959 Thomas 1 423/31 1 3,210,154 10/1965 Klein ct a1.423/307 3,233,968 2/1966 Kocbner et a1. 423/314 3,541,172 11/1970 Stoweet a1. 252/437 X 3,781,222 12/1973 Weisang et a1. 252/437 PrimaryExaminer-Patrick P. Garvin Attorney, Agent, or FirmCurtis, Morris &Safford ABSTRACT Diol dehydration catalysts and methods of preparing andusing the same, constituted at least in part of a pyrophosphate of atleast one metal belonging to the group consisting of lithium, sodium,strontium, and barium and possibly of at least one orthophosphate ofsaid metals and including chromium in combined form and/or a basicadditive.

6 Claims, 24 Drawing Figures PATENTEB JAN 2 8 I975 SHEET 2 OF 8 P/JEPHEUJAN 2 81975 3, 862,964 SHEET 7 [1F 8 F7596 F/G.1Oc1 100A I F/Ci'lQbF/G.']OC

SHEET 8 OF 8 NE 1 xl lll AZ 056E txwt DEHYDRATION CATALYSTS,PARTICULARLY FOR THE DEHYDRATION OF DIOLS This application is aeontinuation-in-part of application Ser. No. 132,892, filed Apr. 9,l97l, now issued as US. Pat. No. 3,78l,222, and the latter is herebyincorporated herein in its entirety by reference.

The present invention relates to catalysts for the dehydration oforganic compounds. More particularly, it concerns catalysts for thedehydration of diols, which diols may or may not be vicinal. Thesedehydration catalysts as well as their preparation constitute variantsof the catalysts and catalyst preparations which form the object ofapplicants aforementioned parent application.

In said application, a dehydration catalyst is described which ischaracterized by the fact that it is formed, in whole or in part, of atleast one mixed or unmixed pyrophosphate, at least one metal belongingto the group formed of:

lithium sodium strontium barium and possibly at least one orthophosphateof at least one metal belonging to the group formed of lithium, sodium,strontium and barium. A process of preparing the catalyst defined aboveis also described in said patent. Finally, the use ofthe catalysts oftheinvention for the dehydration of vicinal or non-vicinal diols is alsodescribed therein.

An object ofthe present application is to describe inventive variants ofthe catalyst and of the process of preparing it, as well as the use ofthe newly obtained catalysts to the dehydration of vicinal ornon-vicinal di- 015.

The applicants have found that chromium in combined form and/or a basicadditive can advantageously be added to the catalyst forming the objectof the parent application, this having the effect of maintaining theactivity and the selectivity of the catalyst with the passage of time,as well as the stability of the catalyst after regeneration.

In the following there will be understood by the selectivity of areaction of the type:

the ratio:

S (Number of mols of B not yet converted into C mols of Cformed)/(Number of mols of A consumed) For example, in the case of thedehydration of a diol into diolefin, it is known that one passes throughan intermediate compound which is an olefin alcohol. The selectivity ofsuch a reaction is therefore given by the ratio:

S (Number of mols of olefin alcohol and diolefin formed)/(Number of molsof diol consumed) With regard to the activity of the catalyst, it isknown that it may be measured by the velocity constant of the reaction AB C: the higher this constant, the greater the activity of the catalyst.lt can therefore be measured by the conversion of the reagent A, definedby the ratio:

C (Number of mols of A B consumed)/(Number of mols of A introduced) Thusfor identical times of contact of the reagent with the catalyst (as withidentical rates of flow of reagent over the same mass of catalyst), theactivities of two different catalysts can be compared.

It is known that it is necessary to regenerate the dehydration catalyst,if it is desired to maintain the activity and the selectivity of thecatalyst at acceptable levels without having to raise the reactiontemperature too much. These regenerations are carried out by calciningthe catalyst in the presence of air and possibly ofsteam.

The stability of the catalyst characterizes its ability to maintain itsselectivity or its activity at satisfactory values with the passage oftime.

A preferred embodiment of the present invention is a dehydrationcatalyst comprising a catalytic composition in accordance with any ofthe claims of the said parent application. and characterized furthermoreby the fact that it contains chromium in combined form and/or a basicadditive.

The chromium may, for instance, be present in the form of chromic oxide(Cr O or in the form of chromium phosphate; the amount of chromiumexpressed as element must be less than or equal to 2% of the totalweight of the catalyst.

Another embodiment of the present invention consists of a process ofpreparing catalysts, which process is characterized by the fact that achromic oxide powder and/or a basic additive in solid state are mixedwith a catalyst composition in accordance with any of the claims of thesaid parent application, that the catalytic mixture obtained is dried,that it is pelletized or extruded in a suitable die, and that thepellets or extrudates thus obtained are calcined.

Another embodiment of the present invention is a process for thepreparation of catalysts, which process is characterized by the factthat mixed or unmixed pyrophosphate, possibly orthophosphate, of a metalbelonging to the group consisting of lithium, sodium, strontium andbarium, and chromium phosphate are coprecipitated from a single aqueoussolution before the shaping and calcining; a basic additive in solidform may possibly be added before the shaping.

The precipitating is effected by a process similar to that described inthe said parent application, but in addition one has an initial solutioncontaining a chromium salt, for instance chromic nitrate. Theproportions of the different species are obviously defined by the finalproportions desired.

The applicants have observed that the beneficial effect of the presenceof chromium is felt less when the chromium is present in excessiveamount in the catalyst; the selectivity of the latter is affected priorto the first regeneration. The change in the selectivity is not sudden,but it may be said that beyond an addition of about 5% chromic oxidewhich corresponds to about 2% chromium calculated as element the initialloss in selectivity becomes a drawback.

It has been stated that the catalyst of the invention may contain abasic additive." By basic additive" there is meant here the hydroxides,oxides and basic salts of a metal belonging to the group consisting ofthe alkaline or alkaline earth metals, or mixtures of these compounds.

In the various catalytic mixtures produced by the applicants, theproportions of the basic additive added have varied between 0 and 20% byweight. However, while small amounts of basic additive activate thecatalyst, too large an amount, on the other hand, deactivates it. Theapplicants have found that the activation is effective for porportionsof basic additive in the catalytic mixture ranging from to by weightapproximately.

Upon the mixing of the components of the'catalyst, a lubricant can beincorporated which may, for instance, be napthalene, camphor, stearicacid, or any other product which permits better flow in the pelleting orextrusion die. This lubricant must, however, be capable of beingeliminated by evaporation or sublimation at a sufficiently lowtemperature, without leaving a residue which might orient thedehydration reaction in a different direction; thus, for instance,aluminum stearate will not be used, since it leaves upon calciningtraces of alumina on the catalyst. The percentage of lubricant added isnot critical; one can, for instance, include 0% to 10% by weight oflubricant in the solid mixture. As a matter of fact, this percentagedepends on the machine used for the mechanical treatment. 1n the case ofcertain devices particularly those provided with means capable ofvibrating the descent hop- LII which therefore contains (depending onthe individual case):

90% or 94% pyrophosphate, 5% or 1% basic additive. 5% naphthalene.

was mixed and then pelleted and calcined for a first time for about 2hours at 300C so as to sublimate the naphthalene, and then a second timefor the same pe riod of time at about 500C.

The catalyst pellets thus obtained were tested for dehydration. Thecatalyst test consists in passing methyl 2,3-butanediol over 1 cc ofcatalyst at atmospheric pressure at the rate of l cc/hour, measured inliquid state, so as to dehydrate it to isoprene, at a temperature equalto 350C or 400C, depending on the tests.

The diol conversion and selectivity results are set forth in Table 1below.

The results of Table I show that numerous catalysts obtained by adding abasic additive to the catalyst of the parent application also constitutevery good dehydration catalysts.

TABLE 1 ADDlTlVE LioH.H,o C210 MgO BaO SrO NaOH None K, 60, Na; C0 LiOH.H1O Nnoti Wt.% 5 5 5 5 5 5 0 l l 1 1 Temperature of the catalytic 350350 350 350 350 350 400 400 400 400 400 test (C) conversion of 67.4 93.461.1 90.5 97.7 96.4 100 99.6 100 99.6 100 the diol Selectivity 80.7 81.481.2 80.5 77.6 83.8 80.3 81.7 82.7 82.9 84.1

per for the catalytic mixture the incorporating of lu- EXAMPLE 2 bricantis unnecessary.

The following examples are not limitative. They illus trate specificembodiments of the preparation of catalysts in accordance with thepresent invention and their use in dehydration reactions.

The accompanying drawings are also given by way of illustration and notof limitation and merely illustrate the results of the tests reported inthe examples.

In the drawings:

FlGS. 1,2 and 3 are graphs of the results of the tests reported inExample 3,

FIGS. 4, 5 and 6 are graphs of the results reported in Example 4,

FIGS. 7a, 7b, 70, 8a, 8b, 8c, 90, 9b, 90, 10a, 10b, 10c, 11a, 11b, llc,12a, 12b, and 120 are graphs of the results of the tests reported inExample 7.

EXAMPLE 1 A double pyrophosphate of lithium and sodium was prepared fromaqueous solutions of LiOH and Na P O by the method of Example 1 of theparent application. This double pyrophosphate was dried at about 110Cand then calcined at 500C. The resultant mass was crushed and sodiumorthophosphate Na HPO.,l2H O was added to it in a weight ratio of:

(Weight of pyrophosphate)/(Weight of hydrated orthophosphate) 3/2 Byaddition of an equal weight ofwater to the powder there was obtained apaste which was dried at about 110C in a oven and then crushed. With thecrusher used, a particle size of 0.2 to 0.5 mm was obtained.

To the particles thus obtained 5% or 1% by weight of various basicadditives indicated in Table 1 were added. Furthermore, 5% by weight ofnaphthalene was added to serve for lubrication. The resultant mixture,

This example is intended to show the importance of the calciningtemperature and its influence on the catalytic properties of variousmixtures prepared by the method of Example 1. Only the temperature ofthefinal calcining changes.

The catalytic tests are carried out in the same manner, using as acharge methyl 2,3-butanediol which is passed over 1 cc of catalyst at350C at atmospheric pressure, at the rate of l cc/hour (rate of flowmeasured in liquid state).

The conversions and selectivites for different calcin ing temperatures,with or without additive, will be found in Table 11.

This example illustrates the use ofa catalyst in accordance with thepresent invention containing 1% caustic soda in accordance with themethod of Example 1 above, for the continuous dehydration of methylbutane- 2,3-diol to isoprene for a long period of time. By

way of comparison, there is described a test carried out i in similarfashion using the preceding catalyst which has not been treated by abasic additive.

A mixture of methyl butane-2,3-diol and 2-methyl l-butene 3-ol (formedof 30 mol% diol and 70 mol% alpha-olefin alcohol) is passed in a reactorover 40 cc of the catalyst (in particles of l to 2 mm), at atmosphericpressure and a temperature which varies in the course of time, and at arate of 40 cc/hour (rate of flow measured in liquid state).

When the molar percentage of isoprene formed in the efflux becomes toolow, it is readjusted by increasing the dehydration temperature. It isknown that the ease of dehydration increases with the temperature. Thisexplains the appearance of temperature shoulders in the accompanyingfigures in which there is plotted, as a function of time:

1. in the case of the dehydration of the diol on the catalyst withoutadditive:

FIG. 1, curve 3/the variation of the average temperature of the reactionFIG. 2, curve l/the molar percentage of isoprene in the efflux FIG. 3,curve S/the conversion of the diol 2. in the case of the dehydration ofthe diol on the catalyst treated with 1% by weight caustic soda:

FIG. 1, curve 4/the variation of the average temperature of the reactionFIG. 2, curve 2/the molar percentage of isoprene in the efflux FIG. 3,curve 6/the conversion of the diol From these various curves, it isconcluded that with the catalyst containing 1% by weight caustic soda,under the conditions of the test:

the molar percentage of isoprene in the efflux from the reactor ishigher the temperatures at which one operates are definitely lower theconversion of the diol is maintained in the vicinity of 100 duringoperation, an identical increase in temperature causes a higher gain inisoprene the temperature shoulders are longer EXAMPLE 4 This exampleillustrates the application of catalysts in accordance with the presentinvention to the continuous dehydration of methyl butane-2,3-diol intoisoprene for a prolonged period of time.

The test was carried out in the same manner as in Example 3, with thesame charge and with the same quantities.

Only the catalyst was different. It contained 2% of a basic additivewhich is caustic soda.

In FIGS. 4 and 5 there are plotted as a function of time:

in curve 7: the temperature of the reaction,

in curve 8: the molar percentage of isoprene in the efflux of thereactor.

In FIG. 6 there has been simply shown the variation with time of theconversion of the diol (curve 9).

Examples 5 and 6 are intended to show that the catalysts in accordancewith the invention are less sensitive to calcining (which operation isnecessary for the regeneration of the catalyst) than catalysts which donot contain chromium in combined form.

Example 7 describes a test on the dehydration of a vicinal diol (methylbutane-2,3-diol) in the presence of a catalyst in accordance with thepresent invention.

EXAMPLE 5 In the present example, the chromium is added in the form ofchromic oxide before the pelleting ofthe catalsyt.

A catalyst 5 is prepared in the following manner:

10.3 liters of a boiling aqueous solution containing 875 g ofmonohydrated lithia are poured into 5.2 liters of a boiling aqueoussolution of sodium pyrophosphate decahydrate containing 2.3 kg ofpyrophosphate Na,P- 2O7'10H2O.

When the supernatant liquid reaches a temperature of 50C, filtration iseffected, followed by washing with 25 liters of water and then drying ata temperature of 120C. 30 g of the dried product, screened to a size ofless than 0.2 mm, and 20 g of disodium orthophosphate (Na HPO -l2H O),screened to a size of less than 0.2 mm, are then mixed together. Themixture is wetted with 50 g of water so as to form a paste which is thendried at a temperature of 120C. The solid obtained is crushed untilobtaining particles of a diameter of between 0.2 and 0.5 mm.

To 40 grams of these grains there are added 0.8 g of chromic oxide (Cr Owhich has been previously calcined at a temperature of ll00C for 16hours), 2 g of naphthalene, and 2 g of didecyl adipate. They are mixedtogether and then pelleted. The catalyst thus obtained will hereinafterbe referred to as catalyst 5.

Catalyst 5 is then separated into two lots, one of which is subjected tocalcining for 2 hours at a temperature of 550C, while the other issubjected to calcining for hours at 550C. The chromic oxide content ofcatalyst 5 is equal to 2%.

A catalyst 5' containing 5% chromic oxide is prepared by a processsimilar to that used for the preparation of catalyst 5 but with a largeraddition of chromic oxide. Moreover, the calcining of the first lot ofcatalyst 5 had been effected at 500C for 2 hours.

A Catalyst T5 containing no chromic oxide was prepared by a processsimilar to that used for the preparation of catalyst 5, but withoutaddition of chromic oxide.

Each of the two lots of catalysts 5, 5' and T is subjected to a methylbutane-2,3-diol dehydration test 1 cc, measured in liquid state, ofpurediol is passed per hour over 1 cm of catalyst at a temperature of 350C.

The results obtained with catalysts 5, 5' and T have been entered inTable III below, in which there have been entered the conversion of thediol and the selectivity of the catalyst in question.

Catalyst 5' was subjected lo calcining for two hours at 500C and not at550C.

The conversion and selectivity of catalyst T decrease much more rapidlythan those of catalysts 5 and 5. It

EXAMPLE 6 In the present example, the chromium is precipitatedsimultaneously with mixed sodium-lithium pyrophosphate. A catalyst 6 isprepared in the following manner:

0.575 liter of a boiling aqueous solution containing 50 grams of lithiamonohydrate and 0.060 liter of a boiling aqueous solution containing22.4. grams of chromic nitrate III nonoahydrate are pouredsimultaneously into 0.350 liter of a boiling aqueous solution containing157.5 grams of sodium pyrophosphate decahydrate.

After cooling to 50C, the precipitate is filtered, washed, and thendried at 120C.

Catalyst 6 is divided into two lots. The first lot is calcined for 2hours at a temperature of 500C. The second lot is calcined for 100 hoursat a temperature of 550C.

A catalyst T containing no chromium is prepared by a process similar tothat used for the preparation of catalyst 6, but the initial solution ofwhich does not contain chromium nitrate.

Each ofthe two lots of catalysts 6 and T is subjected to the catalytictest described in Example 5. The results are entered in Table IV. Thechromium content, expressed as elemental chromium, of catalyst 6 isequal to 2%.

The conclusions which can be drawn are the same as those drawn inconnection with Example 5, with regard to the comparison of theperformances of catalyst T and catalyst 6 of the invention.

EXAMPLE 7 A catalyst 7 is prepared by the following method:

2.3 liters of a boiling solution containing 200 grams of lithiamonohydrate and 0.240 liters of a boiling aqueous solution containing89.6 grams of chromic nitrate nonahydrate are poured simultaneously into1.3 liters of a boiling aqueous solution containing 630 grams of neutralsodium pyrophosphate decahydrate.

The mixture is allowed to cool to 50C whereupon it is filtered and theprecipitate is washed and then dried for 18 hours at 120C.

1 17.55 grams of dried precipitate, screened to a size ofless than 0.2mm, and 58.8 grams of disodium orthophosphate (Na HPO 12 H O), screenedto a size of less than 0.5 mm, are mixed. The mixture is wetted so as toform a paste which is then dried at a temperature of 120C. The solidobtained is crushed until obtaining particles of a diameter of between0.2 and 0.5 mm.

1 gram of Na C0 5 grams of naphthalene, and 5 grams ofdidecyl adipateare added to 100 grams of particles obtained. The substances are mixedtogether and then pelleted. The pellets are then calcined in a stream ofnitrogen, the temperature being slowly increased to 350C, whichtemperature is maintained for 2 hours.

A catalyst T containing no chromium is prepared by a method similar tothat used for catalyst 7 but. on the one hand, the initial solution doesnot contain chromium nitrate and on the other hand, no disodiumcarbonate is added before pelleting.

Catalysts 7 and T are subjected to the following catalytic test: agaseous mixture formed of (in moles) of Z-methyl l-butene 3-ol and 30%of 2-methyl butane 2,3-diol, is passed at a pressure of one atmosphereover 25 cm catalyst with an hourly space velocity of the mixture equalto 1, measured in liquid state. When the molar percent of isoprene inthe efflux (yield of isoprene) becomes too low, the temperature of thereactor is increased.

After 140 hours of operation, catalysts 7 and T are subject to a firstregeneration, which consists in passing a mixture of nitrogen and oxygencontaining 1% oxygen over the catalyst while it is maintained at atemperature of 450C until the efflux no longer contains carbon dioxide.At the end of the regeneration, the temperature is slowly brought to500C.

The catalysts are then used in a second catalytic test identical to thefirst, carried out for 70 hours in the case of catalyst T and hours inthe case of catalyst 7. At the end of this second use, a secondregeneration is effected under the same conditions as the first.Thereupon the catalysts are used for a third time in a catalytic test.

In FIGS. 7a, 7b, 7c, 8a, 8b, 8c, 9a, 9b, 9c, 10a, 10b, 106, 11a, 11b,11c, 12a, 12b, and 12c the results obtained have been shown graphically.

In FIGS. 7a, 7b, and 7c, curves 1, 2 and 3 represent the averagetemperature of the reaction with the use of catalyst T upon the firstuse of the catalyst (curve 1), upon the second use after a firstregeneration (curve 2), and upon the third use after a secondregeneration (curve 3).

In FIGS. 8a, 8b, and 8c, curves 4, 5 and 6 represent the averagetemperature of the reaction with the use of catalyst 7, upon a first, asecond and third use respectively. The letter R designates the momentwhen the use of the catalyst ceased and when a regeneration was theneffected.

In FIGS. 9a, 9b and 9c, curves 7, 8 and 9 represent the conversion ofthe diol introduced into the reactor upon the first, second and thethird use, respectively, of the catalyst T In FIGS. 10a, 10b, 10c,curves 10, 11 and 12 are curves similar to curves 7, 8 and 9, butrelating to catalyst 7.

In FIGS. 11a, 11b and lle, curves 13, 14 and 15 represent the molaryield in isoprene upon the use of catalyst T7.

In FIGS. 12a, 12b and 130, curves 16, 17 and 18 are curves similar tocurves 13, 14 and 15, but relating to catalyst 7.

These curves show the advantages ofcatalyst 7 of the invention overcatalyst T under the conditions of the test. In Table V below, someofthe values which served for the plotting of these curves are indicatedTABLE V lst use 2nd use 3rd use 7 Time (hours) 50 70 50 70 5O Yield ofCatalyst T, isoprene l5 19 3 6 0% Cr Conversion 0% Na,CO of diol 92 8779 60 72 Temp. (C) 342 353 375 386 389 Time (hours) 50 70 50 70 50 70 90Catalyst 7 Yield of 1% Na Co isoprene 22 l8 l6 l3 l5 l2 6 2% CrConversion of diol 98 95 98 98 98 96 85 Temp. ("C) 306 310 320 335 320345 370 A comparison of the performances of the two catalysts in the 3rduse is particularly significant. It is noted that a given yield ofisoprene (6%) is obtained after a longer period of use (90 hours insteadof 50) with higher conversion at a lower temperature in the case ofcatalyst 7, although the second regeneration occurred after a longertime of use. Catalyst 7 is therefore more stable than catalyst T Weclaim:

1. in a dehydration catalyst consisting essentially of at least onemixed or unmixed pyrophosphate of at least one metal chosen from thegroup consisting of lithium, sodium, strontium and barium, theimprovement in which said catalyst further consists of a compound ofchromium chosen from the group consisting of chromium oxide and chromiumphosphate in an amount, expressed as element, which is less than about2% of the total weight of the catalyst.

2. A dehydration catalyst in accordance with claim 1, further comprisingat least one neutral orthophosphate of a metal chosen from the groupconsisting of lithium, sodium, strontium and barium.

3. In a dehydration catalyst consisting essentially of at least onemixed or unmixed pyrophosphate of at least one metal chosen from thegroup consisting of lithium, sodium, strontium and barium, theimprovement in which said catalyst further consists of a basic additivewhich isa compound of an alkaline metal or an alkaline earth metalchosen from the group consisting of the hydroxides, oxides, basic saltsand mixtures thereof, the percentage of said basic additive in thecatalytic mixture finally obtained being less than 10% by weight.

4. A dehydration catalyst in accordance with claim 3, further comprisingat least one neutral orthophosphate ofa metal chosen from the groupconsisting of lithium, sodium, strontium and bariumv 5. In a dehydrationcatalyst consisting essentially of at least one mixed or unmixedpyrophosphate of at least one metal chosen from the group consisting oflithium, sodium, strontium and barium. the improvement in said catalystfurther consisting of at least one basic additive which is a compound ofan alkaline metal or an alkaline earth metal chosen from the groupconsisting of the hydroxides, oxides, basic salts and mixtures thereof,in an amount less than or equal to 10% of the weight of the catalyst anda compound of chromium chosen from the group consisting of chromiumoxide and chromium phosphate in an amount expressed as element less thanor equal to about 2% ofthe total weight of the catalyst.

6. A dehydration catalyst in accordance with claim 5, further comprisingat least one neutral orthophosphate of a metal chosen from the groupconsisting of lithium, sodium, strontium and barium.

2. A dehydration catalyst in accordance with claim 1, further comprisingat least one neutral orthophosphate of a metal chosen from the groupconsisting of lithium, sodium, strontium and barium.
 3. In a dehydrationcatalyst consisting essentially of at least one mixed or unmixedpyrophosphate of at least one metal chosen from the group consisting oflithium, sodium, strontium and barium, the improvement in which saidcatalyst further consists of a basic additive which is a compound of analkaline metal or an alkaline earth metal chosen from the groupconsisting of the hydroxides, oxides, basic salts and mixtures thereof,the percentage of said basic additive in the catalytic mixture finallyobtained being less than 10% by weight.
 4. A dehydration catalyst inaccordance with claim 3, further comprising at least one neutralorthophosphate of a metal chosen from the group consisting of lithium,sodium, strontium and barium.
 5. In a dehydration catalyst consistingessentially of at least one mixed or unmixed pyrophosphate of at leastone metal chosen from the group consisting of lithium, sodium, strontiumand barium, the improvement in said catalyst further consisting of atleast one basic additive which is a compound of an alkaline metal or analkaline earth metal chosen from the group consisting of the hydroxides,oxides, basic salts and mixtures thereof, in an amount less than orequal to 10% of the weight of the catalyst and a compound of chromiumchosen from the group consisting of chromium oxide and chromiumphosphate in an amount expressed as element less than or equal to about2% of the total weight of the catalyst.
 6. A dehydration catalyst inaccordance with claim 5, further comprising at least one neutralorthophosphate of a metal chosen from the group consisting of lithium,sodium, strontium and barium.